1. Field of the Invention
This invention relates to engine control systems and, in particular, to a method and system for controlling air charge motion in the cylinders of a direct injection spark ignition (DISI) engine.
2. Discussion of Related Art
In a DISI engine, the fuel injection nozzle is located inside the combustion chamber rather than the induction pipe as in conventional multi-port or throttle body fuel injection engines. This allows a DISI engine to form a stratified air charge composition in the engine cylinders and to burn air-fuel mixtures having air-fuel ratios that deviate substantially from the stoichiometric air-fuel ratio (14.7:1). DISI engines also have improved thermal efficiency and reduced engine knock as compared to conventional multi-port or throttle body fuel injected engines.
DISI engines are capable of operating in a plurality of different combustion modes including a homogenous combustion mode, a stratified combustion mode, and a hybrid combustion mode. In a homogenous combustion mode, a homogenous air-fuel mixture is present within a cylinder during a combustion event. In a stratified combustion mode, a stratified air-fuel mixture is present within the cylinder. Depending upon the mode of operation, the air charge composition and air charge motion must be adjusted to optimize the combustion process.
In most conventional DISI engines, a swirl control valve actuated by a stepper motor is used to control air charge motion. These conventional engines have a significant drawback. During a transition between combustion modes, the air charge composition changes. For example, when transitioning from a stratified combustion mode to a homogenous combustion mode, there is typically an air-fuel ratio gap (e.g., from  greater than 22:1 to  less than 20:1). As a result, a step change in the fueling rate frequently occurs at the switching instant between combustion modes. The step change in the fueling rate can cause a significant torque disturbance.
The inventors herein have recognized a need for a method and system for controlling air charge motion in a cylinder of an internal combustion engine during a transition between two combustion modes that will minimize and/or eliminate one or more of the above-identified deficiencies.
The present invention provides a method and system for controlling air charge motion in a cylinder of an internal combustion engine during a transition between first and second combustion modes.
A method in accordance with the present invention includes the step of: positioning an intake valve for the cylinder in a first position in advance of the transition using a cam profile switching device. For example, during a transition from a stratified combustion mode to a homogenous combustion mode, the intake valve may be placed in a long valve lift position to hasten egress of air from the engine""s intake manifold. The method may further include the step of moving the intake valve to a second position using the cam profile switching device when a predetermined condition for transitioning between the first and second combustion modes is met. Continuing with the above example, the cam profile switching device may move the intake valve to a short valve lift position to reduce air induction into the cylinder.
A system in accordance with the present invention may include a cam profile switching device coupled to an intake valve for the cylinder. The system may further include an electronic control unit configured to control the cam profile switching device to position the intake valve in a first position in advance of the transition and move the intake valve to a second position when a predetermined condition for transitioning between the first and second combustion modes is met.
A system and method in accordance with the present invention is advantageous. Control of the cylinder intake valve using a cam profile switching device allows changes in valve lift during a single engine event. As a result, air charge motion can be directly and more quickly controlled than by using a swirl control valve or electronic throttle. The transition between combustion modes is therefore accomplished more quickly and LNT purge efficiency and fuel economy is improved. Further, a step change in the air charge can be effected during the transition between combustion modes thereby avoiding the need for a step change in the fueling rate and reducing torque disturbance and increasing drivability performance.
These and other advantages of this invention will become apparent to one skilled in the art from the following detailed description and the accompanying drawings illustrating features of this invention by way of example.